Honestly I think function folders is the wrong solution here. I see two different modes of exploring the code here - a high level over view of what is available and a detailed look at the actual code. Code folding to switch between these two modes is not, I think, the best way to do this. Just the easiest thing to replicate in most editors.

A better solution would be a separate view for these - maybe a side bar or overlay that you can popup when you want to navigate the code.

Rust docs has this - a summary of the methods and other symbols on the side with full descriptions in the main view.

Helix has a nice symbol picker which with some tweaks could be a much nicer way to do this:

If it did not strip so much info from the symbols it would basically show the collapsed view along side the code with the ability to search and jump to the code you are interested in. I want to see more refinement on features like this and not just have code folding which I tend to find more annoying and limiting - having to constantly collapse and expand sections when what I really want is to jump around the code base.